This study of magnetic minerals in a weathering profile developed on plateau basalts of the subtropical southern Paraná Basin explores the evolution of titanomagnetite to titanomaghemite. Six samples studied by optical microscopy, X-ray diffraction (XRD), electron microprobe, Mössbauer spectroscopy (MS) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) support the interpretations.
The profile studied has two major parts: an upper, porous red-clay Latosol, ~2–8 m deep, separated by a stone line from an underlying alterite which has two different facies – its argillaceous alterite consists of a clayey matrix with a well-developed fissure system whereas the underlying boulder alterite consists of rock cores surrounded by highly-porous cortexes of Al-goethite.
Optical microscopy showed the titanomagnetite-titanomaghemite changes in color and shape through the profile. The decrease in the lattice parameter a of the magnetic separates from the rock cores to the alterite facies was detected by XRD. Mössbauer spectroscopy identified non-stoichiometric magnetite in the rock cores and Ti-substituted maghemite in the argillaceous alterite. Chemical analysis of the titanomagnetite-titanomaghemite grains showed that the relative proportions of TiO2 and Fe2O3 vary in the different weathering facies. By SEM and EDS we also detected the presence of minor components as Si, Al, Ca, K, Mg and Mn.
These results led to the interpretation that the titanomagnetites from the fresh Paraná basalts, located in the subtropical zone of Brazil, are unstable and gradually change to titanomaghemites. The evolution of these magnetic minerals is registered in the weathering facies related to climatic changes throughout geological time.